In the field of oil well rod pumping systems, it is desirable to monitor the strain (or “load”) on the polished rod. In this way, malfunctions in the system can be detected and resolved to protect an oil pump from damage. Various solutions have been proposed for how to measure this force on the polished rod. For example, U.S. Pat. Pub. No. 2010/0020808 to Lawson et al. proposes two separate load cells that are “stacked” from the top of the polished rod and respectively measure an upward and a downward force on the polished rod. In order to measure the total force acting on the polished rod, the load cells encircle the polished rod in a ring shape, and are stacked from the top of the polished rod at installation.
Lawson describes an example of a load cell that completely encircles a circumference of the polished rod, and that must therefore be installed from the top of the rod. Such load cells are often used in the field of rod pumping systems. For example, FIG. 2B illustrates a rod pumping system in which a load cell 150 has been installed on polished rod 110 between clamp 130 and clamping bar 140.
Some systems replace the load cell 150 with a load cell that is easily removable from the polished rod 110 and that may, for example, wirelessly transmit load cell data (see, for example, U.S. patent application Ser. No. 14/272,105). However, if a load cell 150 that completely encircles the polished rod is already installed, as illustrated by example in FIG. 2B, removal of the load cell 150 may first need to take place before a different load cell can be installed. Because the load cell 150 is stacked from the top of the polished rod and is positioned between clamp 130 and clamping bar 140, removal can be difficult and time-consuming, and may not always be feasible.
Also, the position of a polished rod (which may correspond to the angle of the walking beam) can be an important metric for determining proper operation of a rod pumping system. Inclinometers are used in rod pumping systems to measure the angle of a walking beam, often by way of such position sensing. For example, an inclinometer installed on a beam of a polished rod pumping system may be used to determine the running distance of the polished rod during pumping operation. Based on the polished rod running distance, information on the pump running status can be obtained.
Existing inclinometers are typically powered by an external power supply, which is often an independent power source needing regular replacement and maintenance. U.S. Pat. No. 7,219,723 to Barnes shows an example of an external power supply. Because the power supply is external to the inclinometer, resulting wires are easily damaged due to the environment (e.g., water and heat). Water can also leak into the inclinometer at the places where the external wires are connected to the inclinometer. Furthermore, installation of the inclinometer is difficult due to the separate external power supply and other external components. Attaching such an inclinometer to the walking beam can be awkward, but placement should be precise in order to obtain accurate measurements.
Furthermore, in “wired” load cells such as those described above, wire(s) connect the load cell to the rest of the pump unit in order to transmit the load signal. However, these wires can be broken by the wind and/or mechanical stress.